I have a high level of skepticism about performance-enhancing supplements. Given the endless succession of supplements being tested, chance dictates that there will be a stream of seemingly positive studies. Few of these studies turn out to be reliably repeatable, so I generally ignore them.
That's why I was very skeptical  when the first studies on beet juice and endurance emerged back in 2009. Subsequent results have shown that the benefits of beet juice are actually real (though there are still questions about the extent to which elite athletes benefit). So I'm inclined to pay a little more attention when the same research group, led by Andy Jones at the University of Exeter, makes new performance claims.
The new study, which appeared a few months ago in the Journal of Applied Physiology , compares two supplements with the potential to have similar effects to beet juice. The active ingredient in beet juice is nitrate, which is converted within the body to nitrite and then to nitric oxide, which is what seems to provide the performance boost. But the body has other ways of producing nitric oxide, including making it from an amino acid called arginine.
Lots of researchers, including Jones and his colleagues , have tried giving subjects arginine to see if it boosts performance, with inconsistent results. Part of the problem is that when you ingest arginine orally, much of it is broken down by your intestinal bacteria and liver. Only about one percent of ingested arginine ends up being used to make nitric oxide.
In the new study, the researchers compared arginine to a placebo and to another amino acid, citrulline. Citrulline is actually one of the byproducts produced when arginine is coverted to nitric oxide, but it's then recycled back into arginine to produce more nitric oxide. The advantage of citrulline is that it can make it through the intestine and past the liver unscathed, unlike arginine—so it may represent a better way of boosting nitric oxide levels in the body.
The study involved 10 volunteers who took the supplements for seven days at a time, and did exercise tests on the sixth and seventh days. Each subject tried all three supplements at different times. The results were somewhat mixed, but the overall conclusion was that citrulline, but not arginine, improved blood pressure (just like beet juice) and boosted performance in "severe intensity" exercise tests lasting around 10 minutes or less.
For example, here's the average power in the three conditions for a test that involved cycling at a fixed hard pace for six minutes then sprinting all-out for one minute:
The increase in time-to-exhaustion in a test that lasted about 10 minutes was 12 percent, which might correspond to a time-trial improvement of a bit less than 1 percent. There are many, many unanswered questions, such as whether trained athletes would get similar benefits. But it's an interesting preliminary hint that there might be something here, joining some earlier results on citrulline.
Oh, and as for dose: the subjects took six grams per day of L-citrulline powder, mixed in water and blackcurrent cordial to disguise the taste. The name citrulline actually comes from the Latin word for watermelon, so it's natural to wonder whether you could get the same benefits from food. Watermelon juice contains 2.33 grams of citrulline per liter, according to the authors, so you'd need about 2.5 liters of watermelon juice to duplicate the dose from this study. Not particularly practical—but then again, the beet juice trend has spawned a wave of concentrated beet drinks, so who knows?